Global Unsteady Pressure Fields Over Turrets In-Flight

Abstract

Point pressure measurements were performed on a hemisphere-on-cylinder turret in-flight and in the wind tunnel at the University of Notre Dame. Measurements were performed at a total of 36 different locations on the turret. The pressure coefficients (c p) was analyzed versus viewing angle for each Mach number and were compared with previous experiments and theoretical values based on the potential flow. At transonic Mach numbers, the dominant frequencies of the motion of the local shock over the turret were investigated. The turret wake was also investigated and it was shown that the "smiles" cutouts and the local shock can have a "regularizing" effect of decreasing higher frequency fluctuations while increasing low frequency fluctuations. Unsteady forces acting on the turret were computed from the pressure fields and their dependence on the Mach number and turret geometry was discussed.

Airborne Aero-Optics Laboratory - Transonic (AAOL-T)

“…The most discernable difference between the two is that the M = 0.7 case has peaks around St = 0.18 and St = 0.3, while the M = 0.8 case has only one peak near St = 0.15; this lower-frequency dynamics was already observed in Figure 14. Results from pressure measurements on AAOL turret [12] have showed similar peak locations for both M = 0.7 and 0.8. To understand a possible mechanism of this low-frequency dynamics, let us recall that a weaker, intermittent shock was observed on the AAOL at a lower M = 0.65 [3] with a typical frequency of St ~ 0.5.…”

“…Both exhibit a peak near St = 0.15 and fall off after that. This peak has been associated with the movement of the separation line on a hemisphere on cylinder turret for subsonic [11] and transonic [12] flow regimes. As the separation bubble is sensitive to the global environment, which is primarily governed by the separated region downstream of the turret, this single peak in the shock spectra indicates that the shock dynamics is linked to the separation line dynamics.…”

“…To understand a possible mechanism of this low-frequency dynamics, let us recall that a weaker, intermittent shock was observed on the AAOL at a lower M = 0.65 [3] with a typical frequency of St ~ 0.5. Also, St = 0.15 has been associated with the unsteady separation line motion over a wide range of subsonic [11] and transonic [12] Mach numbers. So, while at low transonic speeds the shock dynamics is mostly independent of the separation region dynamics, at higher Mach number the shock becomes strong enough to force a premature separation, effectively coupling or locking-in the shock and the separation line dynamics.…”

“…Since both the shock and flow separation are detrimental to beam quality [1,2,13,14], it is important to gain a better understanding of the interaction between them. This knowledge will enable adaptive feedback mechanisms and flow control that will mitigate unwanted aero-optical effects [12].…”

Conditional Studies of the Wake Dynamics of Hemispherical Turret Using PSP

“…For both Mach numbers, the coefficients exhibit a bi-modal distribution, when they are either largely positive or largely negative, with random switching between. Analysis of the pressure fields collected in flight [33] did not reveal any switching in the first POD mode, and neither was it observed in LES-based numerical simulations of the flow around turrets at subsonic speeds [34,35]. One possible reason for this switching behavior is the presence of the tunnel walls, which might make the symmetric wake topology unstable in favor of the becoming-stable shifted wake topologies.…”